(1) Field of the Invention
The present invention relates to an image forming apparatus, such as a printer or a photocopier, which includes a fixing device. In particular the present invention relates to an art of controlling, during start-up of the fixing device, increase of temperature of a heating member used for thermal fixing.
(2) Description of the Related Art
An image forming apparatus, such as a printer or a photocopier, includes a fixing device for performing thermal fixing of an unfixed image formed on a recording sheet. In order that the fixing device can perform the thermal fixing, a warming-up period is required for a heating member in the fixing device to reach a predetermined target temperature (for example 190° C.).
A warming-up operation of the fixing device may commence from various different temperature conditions of the heating member. For example, if the image forming apparatus has been left for a long period with a power supply thereto set to Off or is in sleep mode, when the warming-up operation commences the heating member may have a low temperature close to room temperature. In contrast, if the power supply to the image forming apparatus is set to Off straight after completing a printing operation or a troubleshooting operation (for example of a paper jam), and then set to On again soon after, when the warming-up operation commences the heating member may have a high temperature close to the target temperature.
Consequently, in order to ensure that a heating amount of the heating member during the warming-up operation is not excessive or insufficient, control of the heating amount during the warming-up operation is performed based on the temperature condition of the fixing device while monitoring surface temperature of the heating member using a temperature sensor. If temperature of the heating member exceeds the target temperature then electrical power (hereinafter simply referred to as “power”) is unnecessarily consumed, and thermal degradation of configuration elements of the fixing device such as the heating member and the temperature sensor is increased, thus reducing life-span thereof. However, the slower the increase of temperature of the heating member, the longer the warming-up period becomes, and thus the longer a waiting period which a user must wait for printing to be performed becomes.
As the temperature sensor, a contact temperature sensor which is in contact with the surface of the heating member, or a non-contact temperature sensor which is not in contact with the surface of the heating member may be used.
A contact temperature sensor is cheaper than a non-contact temperature sensor, but has a disadvantage that contact with the heating member causes friction between the contact temperature sensor and the surface of the heating member, reducing life-span thereof. Another disadvantage is that if toner becomes attached to a part of the surface contacting with the contact temperature sensor, surface temperature of the heating member may be detected through the toner, thus causing a deviation of the detected surface temperature from actual surface temperature of the heating member. In recent years, as a result of the above disadvantages, use of a non-contact temperature sensor, such as thermopile, to detect surface temperature of a heating member has become common (for example refer to Japanese Patent Publication No. 2002-139952).
Once the warming-up operation commences, control of the heating amount of the heating member is performed while monitoring temperature of the heating member using the non-contact temperature sensor, so that the temperature of the heating member does not exceed the target temperature. Specifically, in the above control a heating operation of the heating member is performed when the temperature of the heating member is lower than the target temperature, and the heating operation is suspended when the temperature of the heating member reaches the target temperature. Through the above control, the surface temperature of the heating member can be controlled to increase to the target temperature during the warming-up operation, without the heating amount of the heating member being excessive or insufficient.
On the other hand, the non-contact temperature sensor has a disadvantage as described below. The non-contact temperature sensor is easily influenced by ambient temperature around the non-contact temperature sensor. Therefore, when the ambient temperature is unstable, temperature of the heating member detected by the non-contact temperature sensor may deviate from actual temperature of the heating member. Thus, the disadvantage of the non-contact temperature sensor is that the non-contact temperature sensor cannot accurately measure temperature of the heating member while the ambient temperature is unstable.
The ambient temperature is particularly likely to be unstable when the fixing device transitions from a non-power supplied state to a power supplied state. The above is due to complex air currents being easily created in the image forming apparatus when power supply commences. The complex air currents are induced by operation of a fan for example, and cause instability of the ambient temperature.
As a consequence of the above, even if power supply commences to the fixing device, conventionally control of the heating amount of the heating member is commenced after a waiting period of a certain length (one second for example) has passed and the ambient temperature is stabilized.
Therefore, if control of the heating amount of the heating member is commenced after the waiting period has passed as described above, the warming-up operation of the heating member is delayed by an amount of time equivalent to the waiting period. The above causes a disadvantageous effect of length of a printing waiting period being increased. Alternatively, heating of the heating member could be commenced during the waiting period. However, if the heating member has a temperature close to the target temperature, the above causes a disadvantageous effect of the heating member being excessively heated so as to exceed the target temperature.